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Grain & Graze - Using sorghum case study

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					Grain & Graze National Feedbase Project

Using sorghum as a dual purpose crop by
grazing ratooned stubble in the Border
Rivers region


JPM Whish

18 May 2007
Enquiries should be addressed to:
Dr «GreetingLine»
CSIRO Sustainable Ecosystems
GPO Box 102
Toowoomba 4350
jeremy.whish@csiro.au




Sorghum as a dual purpose crop in the Border Rivers   2
The decision to ratoon grain sorghum after harvest has implications for following crops. Water is the
underlying driver of northern farming systems and the decision to use water for the production of
additional biomass may have implications on yields of the next crop. This analysis attempted to
answer the question: will the water used by a ratooned unsprayed sorghum crop significantly reduce
the starting water for a following crop, resulting in reduced grain yield? What extra sorghum biomass
could be grown from rationed sorghum potentially for livestock production? A simulation analysis
using the APSIM-grain sorghum module and a modified forage sorghum-module were used to assess
the implications of ratooned sorghum biomass on the plant available water for following sorghum
crops.
Allowing October-sown sorghum crops to ratoon after harvest reduced the starting water for a
following October crop by an average of 14 mm at the wetter location of Warialda and 23 mm at the
drier location of Nindigully. On average the production of forage by ratooning sorghum after grain
harvest cost the following sorghum crop 108 kg/ha of grain at Warialda and 332kg/ha of grain at
Nindigully. An estimate of the average additional biomass produced by ratooning the crop was ~ 7121
kg/ha at Warialda and ~6469 kg/ha at Nindigully. This result shows if water is the sole decision
criteria, then in the majority of seasons allowing sorghum to ratoon would significantly impact on the
PAW for following sorghum crops.
Introduction
Divided opinions exist as to the suitability of using sorghum as a dual-purpose crop and grazing
ratooned regrowth after grain harvest. It is argued that allowing sorghum to re-grow depletes stored
soil water reserves potentially limiting the yield of following crops. The counter argument suggests the
lost yield to be insignificant compared to the benefit of supplementing the feedbase. Quantifying the
impact of regrowth on stored PAW will inform and assist in understanding the tradeoffs associated
with grazing sorghum stubble.
Methods
The APSIM simulation model was used to identify how much water was depleted from the soil by
ratooning sorghum. However, the current grain sorghum module in APSIM does not allow grain
sorghum to ratoon. A simplified grain sorghum module based on forage sorghum was developed and
parameterised to produce similar biomass and water use results to grain sorghum. This model was
used in combination with the grain sorghum module to assess the impact of allowing crops to ratoon.
All simulations were performed on a grey vertosol soil with a PAWC for sorghum of 220 mm.
Simulations were run using the Warialda, NSW (650mm mean annual rainfall, located in the east of
the northern cropping region) and Nindigully, Queensland (520 mm mean annual rainfall, located in
the west of the cropping region) daily meteorological records for the years 1957 to 2006. Sorghum
was sown at a crop density of 70,000 plants per ha on a 1m row spacing. Both grain and forage
sorghum were fertilised equally by the addition of 150 kg/ha of urea at sowing, no additional nitrogen
was applied to the ratooning crop.
Results
The forage sorghum module was adjusted to reproduce similar biomass and water use values to the
grain sorghum module (Figs 1-3). However, at this stage the water use by the forage sorghum module
is higher than that of the grain sorghum (Fig. 2).




Sorghum as a dual purpose crop in the Border Rivers                                                   3
Figure 1: Modelled biomass predictions of the existing APSIM-Sorghum module v the modified APSIM-Forage
sorghum module.




Figure 2: Modelled plant available water (PAW) predictions of the existing APSIM-Sorghum module v the modified
APSIM-Forage sorghum module.




Figure 3: Modelled grain yield predictions of the existing APSIM-Sorghum module v the modified APSIM-Forage
sorghum module.




Sorghum as a dual purpose crop in the Border Rivers                                                           4
Grain yield was not reproduced as well, with the new model consistently under-predicting compared
to the grain sorghum module (Fig. 3). The new model successfully ratooned after harvest and
continued to produce biomass into the cooler months; all ratooned sorghum was killed on 1st May. No
validation of the ratooned biomass is available so ratoon biomass simulations are only indicative at
this stage.
Allowing a sorghum crop to ratoon and use water between grain harvest and 1st May produced a
significant difference (P < 0.05) in PAW compared to a non-ratooned control when sampled at winter
crop sowing (15th May) at Nindigully (average 34 mm) and Warialda (average 33 mm) (Figs 4,5).




Figure 4: Box and whisker plots showing the difference in plant available water between grain sorghum and
ratooned sorghum at sorghum harvest (15-20 Jan), winter crop sowing on 15th May and summer crop sowing on
11th October at Warialda.




Figure 5: Box and whisker plots showing the difference in plant available water between grain sorghum and
ratooned sorghum at sorghum harvest (15-20 Jan), winter crop sowing on 15th May and summer crop sowing on
11th October at Nindigully.




Sorghum as a dual purpose crop in the Border Rivers                                                     5
No difference in soil PAW was observed between the ratooned and non-ratooned sorghum crops by
the following October sowing date for crops grown in Warialda (average 14 mm), but a significant
difference existed in Nindigully (average 24 mm).
The impact on the yield of a following grain sorghum crop sown in the following October was not
significant despite back to back grain sorghum crops consistently out yielding grain sorghum
following a ratoon at both sites (Figs 6,7). The ratoon crop had a greater impact on grain sorghum
yields at the drier location of Nindigully.




Figure 6: Difference (grain minus ratoon) in yield between grain sorghum grown at Warialda after a short fallow
and grain sorghum after a ratooned sorghum crop. Grain sorghum was sown on the 15 October.




Figure 7: Difference (grain minus ratoon) in yield between grain sorghum grown at Nindigully after a short fallow
and grain sorghum after a ratooned sorghum crop. Grain sorghum was sown on the 15 October.

On average the production of forage by ratooning sorghum after grain harvest cost the following
sorghum crop 108 kg/ha of grain at Warialda and 332 kg/ha of grain in Nindigully. An estimate of the



Sorghum as a dual purpose crop in the Border Rivers                                                               6
additional biomass produced by ratooning the crop produced on average ~ 7121 kg/ha at Warialda and
~6469 kg/ha at Nindigully
Conclusion
The view that allowing a sorghum crop to ratoon will significantly impact on the starting PAW of
following crops is generally supported by these results especially in the low rainfall area of
Nindigully. In Warialda sufficient rainfall fell over the summer to replace the majority of the water
used by the ratoon. However, despite yields of the following crops being reduced it was not
statistically significant. If the intention is to double crop winter crops after sorghum then the PAW
following a ratooned sorghum will be significantly lower than following a crop sprayed out at harvest
If the decision to ratoon sorghum is singular and only involves the trade-off between water for
biomass and livestock feed or water for the following crop. Then the decision to ratoon will impact on
the next crop but not significantly. However, as with many farming decisions trade-offs are pluralistic
and other considerations will impact on the decision.
Basing a decision simply on available water is valuable, but simplistic. What this analysis does not
consider is the management implications of ratooning sorghum. Such implications are mixed and
varied, but include: the creation of a green bridge for disease and insects, poor weed control, reduced
soil cover, the tie up of nitrogen reserves, and implications on deep drainage and erosion. Future work
will look at some of these components and link these with surveyed responses from farmers examining
the motivation of farm practitioners to ratoon sorghum stubble.




Sorghum as a dual purpose crop in the Border Rivers                                                   7

				
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